D.C. motors are typically energized in a closed loop configuration including an amplifier for providing the required voltage and current to the motor, an encoder for sensing the actual position and velocity of the motor and a controller responsive to a command signal and to the position and velocity signals for generating an error signal. The command signal is supplied from a computer or other system controller and represents the desired motor position. The error signal is supplied to the amplifier and determines the required motor drive. The error signal is typically a bipolar voltage which represents the difference between the desired motor position and the actual motor position, including the sign of the difference.
Linear amplifiers are frequently used for driving d.c. motors. Linear amplifiers are relatively simple and provide a high degree of accuracy in a closed loop system. However, the efficiency decreases at low motor speed since the power not supplied to the motor is dissipated in the amplifier. Pulse width modulated (PWM) amplifier systems have also been used for driving d.c. motors. A variable duty cycle pulse train is supplied to the motor, the duty cycle varying in response to the error. When the error is zero, a fifty percent duty cycle square wave signal is supplied to the motor. PWM amplifiers are more efficient than linear amplifiers since the power transistors are switched on and off rather than operating in a linear region. In the on state, the transistor power dissipation is low because the voltage drop across it is low. In the off state, the transistor current is zero. Most of the power is dissipated during the transitions from on to off and off to on. However, PWM amplifiers are more complex and expensive than linear amplifiers and dissipate power even when the error is zero since a fifty percent duty cycle signal is supplied to the motor.
A stepping motor drive circuit utilizing a chopper power switching bridge circuit and a one-shot circuit for establishing a uniform chopper off time is described by S. Palmin in "Bipolar MOSFET Stepping Motor Drive Design," Power Conversion International, October 1984, pp. 34-37. The disclosed configuration is not applicable to d.c. motor drives since the drive requirements for stepper motors are substantially different from those of d.c. motors.
It is desirable to provide an amplifier system for energizing d.c. motors which overcomes some or all of the above disadvantages. The amplifier system should be compatible with existing controllers so that it can operate directly from conventional error signals.
It is a general object of the present invention to provide an improved amplifier system for d.c. motors.
It is another object of the present invention to provide a d.c. motor amplifier system having high efficiency.
It is a further object of the present invention to provide a d.c. motor amplifier system which is low in cost and easy to manufacture.